The present invention is directed to liquid coating compositions and methods for forming the same that, when applied to a substrate, are useful in forming transparent, abrasion resistant coatings thereon. Such coating compositions can further preferably be formulated such that the refractive index thereof can be adjusted to correspond to the refractive index of the substrate
Previously, transparent coatings have been described for plastic materials where there is a need for abrasion resistance. Exemplary of such coatings include those disclosed in U.S. Pat. No. 4,355,135, issued to James January on Oct. 19, 1982, entitled TINTABLE ABRASION RESISTANT COATINGS; U.S. Pat. No. 6,001,163 issued to Harvey, et al. on Dec. 14, 1999 entitled COMPOSITION FOR PROVIDING AN ABRASION RESISTANT COATING ON A SUBSTRATE; U.S. Pat. No. 6,348,269 issued to Terry on Feb. 19, 2002 entitled COMPOSITION FOR PROVIDING AN ABRASION RESISTANT COATING ON A SUBSTRATE HAVING IMPROVED ADHESION AND IMPROVED RESISTANCE TO CRACK FORMATION, the teachings of all of which are expressly incorporated herein by reference. Likewise known in the art are coating compositions that have been formulated to impart abrasion resistance that further possess a tunable refractive index. Examples of such coatings include those disclosed in U.S. Pat. No. 5,322,888 issued to Kato, et al. on Jun. 21, 1994 entitled COATING COMPOSITION FOR OPTICAL PLASTIC MOLDINGS; U.S. Pat. No. 6,342,097 issued to Terry, et al. on Jan. 29, 2002 entitled COMPOSITION FOR PROVIDING AN ABRASION RESISTANT COATING ON A SUBSTRATE WITH A MATCHED REFRACTIVE INDEX AND CONTROLLED TINTABILITY; U.S. Pat. No. 6,538,092 issued to Terry, et al. on Mar. 25, 2003 entitled COMPOSITION FOR PROVIDING AN ABRASION RESISTANT COATING ON A SUBSTRATE WITH A MATCHED REFRACTIVE INDEX:2, the teachings of all of which are expressly incorporated herein by reference.
Such compositions, however, have been largely limited to using a carboxylic acid and/or a carboxylic anhydride which may or may not be used in combination with a strong or mineral acid for the hydrolysis catalyst. U.S. Pat. No. 4,355,135 specifies the polyfunctional carboxylic acid and polyfunctional anhydride compounds as cross linking components wherein there is preferably equimolar amounts of both the epoxy and carboxylate functional groups to thus enable the carboxylate groups of the coating composition to condense with the epoxy groups of the coating composition. U.S. Pat. No. 6,001,163 and U.S. Pat. No. 6,538,092 describe poly and/or multi-functional carboxylic acids and anhydrides as cross linking components which are thought to provide matrix enhancement. U.S. Pat. No. 4,355,135 specifies, however, that sometimes the polyfunctional carboxylic acid is partially or fully incompatible with the coating matrix. In this regard, such incompatibility becomes apparent during the thermal curing step where the solid carboxylic acid component crystallizes to the surface of the coating matrix. For example, U.S. Pat. No. 5,322,888 describes the incompatibility of the polycarboxylic acid as a concentration effect. While using a lower concentration of a polyfunctional carboxylic acid may solve the incompatibility problem, such lesser concentration of acid will typically restrict the amount of basic condensation catalysts which can be used due to the reduced stability of higher pH compositions.
Accordingly, there is a substantial need in the art for a coating composition that is operative to form a transparent, abrasion resistant coating that can further preferably be formulated to have a tunable refractive index that also eliminates all known deficiencies in the art associated with coatings which incorporate the use of carboxylic acid or carboxylic anhydrides. There is likewise a need in the art for such a coating that further utilizes substantially less material; i.e., acid (of an order of several magnitudes) which in turn provides for a significantly more homogeneous coating matrix. There is still further a need in the art for such a coating that, by virtue of utilizing significantly less material provides for a significantly more homogeneous coating matrix. There is still further a need in the art for such a coating that is of simple formulation, cost-effective to produce and apply, can be utilized in a substantial number of applications, utilizes components that are relatively inexpensive and commercially available, can be deployed utilizing virtually any type of conventional coating deposition process, and further can form an abrasion resistant coating without the presence of a poly or multifunctional carboxylic acid or anhydride cross linking compound to thus eliminate all unfavorable properties associated therewith.